Subpopulations of lymphocytes may be defined via functional, morphological and immunochemical differences. Included among the properties that distinguish thymus-derived (T) vs. bone marrow-derived (B) lymphocytes are the apparent presence or absence of receptor sites for specific substances and the response of the involved cell type to interaction with mitogens. These differences between T and B cells reflect discrepancies in the chemical composition and structural arrangements of the plasma membrane. B cells may be further divided into subpopulations on the basis of surface immunoglobulin. More recently, subpopulations of T cells have been defined on the basis of function and cell surface alloantigens; helper (Ly 1 plus 2 minus 3 minus) and supressor (Ly 1 minus 2 plus 3 plus) cells are of particular importance. Investigations to date suggest that most, if not all, the above subpopulations differ in their sensitivity to ionizing radiation. Thus: 1) irradiated B cells placed in tissue culture survive less well than T cells; 2) the capacity of irradiated cells to traffic in normal fashion is inhibited at lower dose levels for B cells than T cells; 3) after whole body exposure, proportionately greater numbers of T than B cells continue to recirculate and maintain viability; 4) B cells demonstrate radiation-induced morphologic alterations at lower dose levels than do T cells as documented by transmission and scanning electron microscopy; 5) T and B cells activated to appropriate mitogens or antigens in vivo and in vitro are more resistant than their nonactivated counterparts; 6) supressor T cells appear to be more radiosensitive than helper T cells; 7) a subpopulation of cytotoxic T cells is very resistant to radiation injury. The purpose of this project is twofold: expand the above observations on the radiosensitivity of defined populations of lymphocytes with particular attention to low dose effects; define he physical-chemical basis of these differences in radiosensitivity and thereby gain a better understanding of the pathogenesis of radiation-induced interphase death of lymphocytes.